Cyanate ester compounds form triazine rings by curing, and because of their high heat resistance and excellent electrical characteristics, cyanate ester compounds have been conventionally widely used as raw materials of various functional polymer materials, such as structural composite materials, adhesives, electrical insulating materials, and electrical and electronic components. However, in recent years, with an advance in required performance in these application fields, various properties required of functional polymer materials have become increasingly strict. Examples of such properties include flame retardancy, heat resistance, a low dielectric constant, a low dielectric loss tangent, weather resistance, chemical resistance, low water absorbency, and high fracture toughness. However, these required properties have not always been satisfied so far.
For example, in the field of printed wiring board materials, with a higher communication frequency and clock frequency, a lower dielectric constant and dielectric loss tangent have been required. Therefore, in recent years, cyanate resins having excellent dielectric characteristics have been widely used.
In addition, from the viewpoint of ensuring safety against fire, bromine compounds having high flame retardancy are used in order to provide flame retardancy. As such bromine compounds, for example, brominated bisphenol A (see Patent Literature 1), a glycidyl ether of brominated phenol novolac (see Patent Literature 2), brominated maleimides (see Patent Literature 3), monofunctional cyanate ester compounds having bromine (see Patent Literature 4), and addition bromine compounds having no reactivity with cyanate ester compounds (see Patent Literature 5) are known.
However, although these bromine compounds have high flame retardancy, there is a possibility that they can not only form corrosive bromine or hydrogen bromide by pyrolysis, but form bromine compounds having strong toxicity associated with the dioxin problem in recent years, when decomposing in the presence of oxygen. Therefore, materials not containing these bromine-based flame retardants are required.
Therefore, as flame retardants replacing bromine, phosphorus-containing compounds, and nitrogen or sulfur-containing compounds are studied. For example, as phosphorus-containing compounds that are often blended in epoxy resins, triphenyl phosphate, resorcinol bis(diphenyl phosphate), and the like are known.
However, when large amounts of these phosphorus-containing compounds are blended in resin compositions, the heat resistance, moisture resistance, water absorbency, and the like of the resin compositions are often deteriorated. In order to improve this, using phosphorus-containing compounds having a phenolic hydroxyl group in combination with divalent cyanate ester compounds is also studied (for example, see Patent Literature 6, Patent Literature 7, and Patent Literature 8). But, for use of phosphorus-containing compounds, the problem of toxicity is concerned as for the above bromine compounds. Moreover, phosphorus-containing compounds are often difficult to landfill, and generation of a phosphine gas during combustion is also concerned. In addition, as nitrogen-containing compounds, melamine, guanidine, and the like are known, but the flame retardancy is insufficient when they are used alone.
On the other hand, as other components that provide flame retardancy to resin compositions, metal hydroxides, such as aluminum hydroxide and magnesium hydroxide, are known. However, blending of metal hydroxides may cause a deterioration in the dielectric characteristics, heat resistance, impact resistance, and moldability of resin compositions. In addition, for example, attempts are also made to use large amounts of inorganic fillers, such as spherical fused silica, as used in epoxy resins to decrease combustible components, thereby ensuring flame retardancy. However, there are fears that highly blending inorganic fillers increases the melt viscosity of resin compositions, causes a deterioration in moldability, and a deterioration in adhesion due to a deterioration in wettability on a substrate, and further, worsens dielectric characteristics, and the like. In addition, there are also attempts to use antimony-based flame retardants, such as antimony trioxide, in combination with brominated epoxy resins. However, the antimony-based flame retardants are generally toxic substances, and therefore, there is a fear of chronic toxicity.
From the viewpoints as described above, an improvement in the flame retardancy of functional polymer materials themselves blended in resin compositions is required more than ever.
In addition, in order to improve heat resistance, a low dielectric constant, a low dielectric loss tangent, weather resistance, chemical resistance, low water absorbency, high fracture toughness, moldability, adhesiveness, and the like simultaneously with flame retardancy, many attempts have been made so far. For example, a method for making a cured product having excellent thermal stability by combining a monofunctional cyanate ester compound with a bifunctional cyanate ester compound (see Patent Literature 9), and a method for promoting a lower dielectric constant and a lower dielectric loss tangent by combining a monofunctional cyanate ester compound with a polyfunctional cyanate ester compound (see Patent Literature 10) are known.
In addition, a method for producing a flame-retardant cyanate curing resin composition having low moisture absorbency, while promoting a lower dielectric constant and a lower dielectric loss tangent, by adding a monofunctional cyanate ester compound containing a halogen is known (see Patent Literature 4). This Patent Literature 4 describes various cyanate ester compounds, but use of an aromatic monofunctional cyanate ester compound having bromine as a functional group is essential for maintaining flame retardancy, and improving flame retardancy without using bromine is not successful.
Further, Patent Literature 11 describes a method for promoting flame retardancy using an aromatic cyanate ester compound containing at least two rings bonded by a group containing an unsaturated group, Patent Literature 12 describes a method for promoting flame retardancy using a fluorine-containing dicyanate ester compound, and Patent Literature 13 describes a method for promoting flame retardancy using a phenol novolac-based cyanate ester compound.
However, in these methods, a cured product of a single practical cyanate ester compound having all performance of dielectric characteristics, flame retardancy, and heat resistance, at high levels is not obtained.
On the other hand, it is known that a cured product of a cyanate ester compound having an aralkyl structure has low dielectric characteristics, high flame retardancy, and high heat resistance (see Patent Literature 14).